A. Field of the Invention
The present invention relates to a perpendicular magnetic recording medium mounted on various magnetic recording devices, to a method of manufacturing the same, and to a magnetic recording device using the perpendicular magnetic recording medium.
B. Description of the Related Art
A perpendicular magnetic recording system in which the recording magnetization is recorded in a direction perpendicular to the plane of a medium is drawing attention as a technology to attain high density of magnetic recording in place of a conventional longitudinal magnetic recording system in which the recording magnetization is recorded in a direction along the plane of a medium. A perpendicular magnetic recording medium is mainly composed of a magnetic recording layer of a hard magnetic material with perpendicular magnetic anisotropy, an underlayer for aligning the magnetic recording layer in the desired direction, a protective layer for protecting a surface of the magnetic recording layer, and a backing layer of a soft magnetic material for concentrating the magnetic flux generated by a magnetic head used to record on the recording layer. This medium usually has a layer structure in the sequence of backing layer/underlayer/magnetic recording layer/protective layer. Although a soft magnetic backing layer raises the medium performance, this layer is occasionally omitted since recording is possible without the soft magnetic backing layer. A medium without a soft magnetic backing layer is called a single layer perpendicular magnetic recording medium (hereinafter “a single layer perpendicular medium”), while a medium with a soft magnetic backing layer is called a double layer perpendicular magnetic recording medium (hereinafter “a double layer perpendicular medium”).
A perpendicular magnetic recording medium (hereinafter “a perpendicular medium”), similarly to a longitudinal magnetic recording medium, also needs compatibility between low noise and high thermal stability to achieve high recording density. Low noise can be achieved by minimization of magnetic grains or reduction of magnetic interaction between the magnetic grains (hereinafter “intergranular interaction”). There is an index called a magnetic cluster size that includes an effect of a magnetic grain diameter and represents a magnitude of the intergranular interaction. A magnetic cluster is composed of a plurality of magnetic grains. The magnetic cluster size is small when the size of the magnetic grain or the intergranular interaction is small. Therefore, the magnetic cluster size must be decreased for the noise reduction.
However, a limitation is imposed on the minimization of the magnetic grains composing the magnetic recording layer because the magnetic grains are transformed to a superparamagnetic state and lose ferromagnetic property when the grain diameter becomes smaller than 4 nm. Accordingly, the reduction of the intergranular interaction is a key issue for the enhancement of recording density. Thus, a patterned media has been proposed aiming at an ideal condition in which one grain is equal to one bit by forming one grain in a magnetic layer in a truncated cone shape applying a technique in the semiconductor process. (See Abstracts of the papers presented at 24th meeting of The Magnetic Society of Japan, p. 283 (2000), for example.)
In a perpendicular medium having a magnetic recording layer composed of normal continuous films, the formation of a segregation structure has been proposed in which a CoCr alloy is used and the concentration of nonmagnetic chromium in the grain boundary is at least 1.4 times the concentration in the crystal grains. (See Japanese Unexamined Patent Application Publication No. 2002-358615, for example.) In addition, a magnetic recording layer (called a granular magnetic recording layer) has been proposed that employs a grain boundary phase of a nonmagnetic and nonmetallic substance, for example, oxide or nitride. (See Japanese Unexamined Patent Application Publication No. H3-58316 and U.S. Pat. No. 5,679,473, for example.)
In every above-mentioned proposal, an appropriate underlayer is used for the purpose of controlling crystal alignment of the magnetic recording layer. However, most attempts for improving the isolation in the magnetic recording layer, i.e., decreasing the intergranular interaction, are currently carried out by changing the composition of the magnetic recording layer itself or improving the process of depositing the layer. Among these, it has been proposed to miniaturize the grains in a magnetic layer utilizing an effect of an underlayer in a perpendicular medium employing a magnetic layer of a multilayer film of artificial lattice. (See Japanese Unexamined Patent Application Publication No. 2003-223712, for example.) The miniaturization of the grains in a magnetic layer was difficult in this type of magnetic layer material. The miniaturization was carried out in the reference by forming the crystal grain in the underlayer in a sphere or an ellipsoid, to produce irregularity on the underlayer surface.
To magnetically isolate magnetic grains in a magnetic recording layer and decrease the intergranular interaction, it is effective to form an underlayer having a structure of physically isolated crystal grains and isolate magnetic grains in the magnetic recording layer utilizing the effect of the underlayer. A method similar to the one disclosed in Japanese Unexamined Patent Application Publication No. 2002-358615, for example, possibly forms an underlayer having an artificially isolating structure. The method is, however, very complicated in the manufacturing process, and thus unsuitable for mass production.
The use of an underlayer having an irregular surface produced by crystal grains as proposed in Japanese Unexamined Patent Application Publication No. 2003-223712, also has a problem. The shape of the surface of crystal grains in the underlayer affects the dispersion of crystal orientations in the magnetic recording layer, and the dispersion of the crystal orientations, in turn, means the dispersion of orientations of perpendicular magnetic anisotropy. Thus, a large curvature of the surface of crystal grains on the substrate surface is considered to degrade the dispersion of orientations of perpendicular magnetic anisotropy. From this point of view, the shape of the surface of crystal grains in the underlayer, which is reflected in the magnetic recording layer disposed on the underlayer, is preferably made as flat as possible. When the grain diameter of the crystal grains in the underlayer is increased for decreasing the curvature of the grain in this method, the number of grains per unit area decreases. Because the number and size of the magnetic grains in the magnetic recording layer reflect the number and size of the crystal grains in the underlayer, a problem of thermal fluctuation becomes significant due to decrease of the number of magnetic grains. On the other hand, when the grain diameter of the crystal grains in the underlayer is decreased, the grain diameter of the magnetic grains in the magnetic recording layer decreases reflecting the decrease of the area of a crystalline portion per one crystal grain on the underlayer surface. As a result, grains smaller than 4 nm, which are transformed to a superparamagnetic state, increase and this also raises the problem of thermal fluctuation.
Therefore, the underlayer proposed in Japanese Unexamined Patent Application Publication No. 2003-223712, while allowing minimization of magnetic grains, cannot simultaneously satisfy all of the following conditions:
(1) to increase the number of magnetic grains per unit area,
(2) to suppress generation of superparamagnetic grains, and
(3) to reduce the dispersion of orientations in the magnetic recording layer.
Further, when the crystal grains in the underlayer is minimized for the purpose of minimization of the magnetic grains, improvement in crystallinity of the underlayer is difficult since the volume of a crystalline portion is small particularly at the initial stage of growth, which is essential to obtain the crystallinity. As a result, a relatively large thickness is needed to accomplish sufficient crystal growth. For the above-described reason, it is difficult to improve medium performance or enhance recording density by using an underlayer comprising crystal grains having the shape of a sphere or an ellipsoid.
Accordingly, there remains a severe problem to obtain a method of manufacturing such a perpendicular medium that an isolation structure of an underlayer can be formed using a simple method as in a conventional manufacturing process of a medium of continuous films and the isolation structure of underlayer allows suppressing the dispersion of alignment and reducing a magnetic cluster size in a magnetic recording layer, and that a high recording density can be achieved owing to thin film thickness of the underlayer.
The present invention is directed to overcoming or at least reducing the effects of one or more of the problems set forth above.